So, you've set up a forum on your site for the first time, or enabled comments on your blog. You carefully craft a post or two, click the submit button, and wait with bated breath for comments to come in.

And they do come in. Perhaps you get a friendly note from a fellow blogger, a pressing update from an MMORPG guild member, or a reminder from your Aunt Millie about dinner on Thursday. But then you get something else. Something... disturbing. Offers for deals that are too good to be true, bizarre logorrhean gibberish, and explicit images you certainly don't want Aunt Millie to see. You are now buried in a deluge of dreaded comment spam.

Comment spam is bad stuff all around. It's bad for you, because it adds to your workload. It's bad for your users, who want to find information on your site and certainly aren't interested in dodgy links and unrelated content. It's bad for the web as a whole, since it discourages people from opening up their sites for user-contributed content and joining conversations on existing forums.

So what can you, as a webmaster, do about it?

A quick disclaimer: the list below is a good start, but not exhaustive. There are so many different blog, forum, and bulletin board systems out there that we can't possibly provide detailed instructions for each, so the points below are general enough to make sense on most systems.

Make sure your commenters are real people

Add a CAPTCHA. CAPTCHAs require users to read a bit of obfuscated text and type it back in to prove they're a human being and not an automated script. If your blog or forum system doesn't have CAPTCHAs built in you may be able to find a plugin like Recaptcha, a project which also helps digitize old books. CAPTCHAs are not foolproof but they make life a little more difficult for spammers. You can read more about the many different types of CAPTCHAS, but keep in mind that just adding a simple one can be fairly effective.

Block suspicious behavior. Many forums allow you to set time limits between posts, and you can often find plugins to look for excessive traffic from individual IP addresses or proxies and other activity more common to bots than human beings.

Use automatic filtering systems

Block obviously inappropriate comments by adding words to a blacklist. Spammers obfuscate words in their comments so this isn't a very scalable solution, but it can keep blatant spam at bay.

Use built-in features or plugins that delete or mark comments as spam for you. Spammers use automated methods to besmirch your site, so why not use an automated system to defend yourself? Comprehensive systems like Akismet, which has plugins for many blogs and forum systems and TypePad AntiSpam, which is open-source and compatible with Akismet, are easy to install and do most of the work for you.

Nofollow untrusted links. Many systems have a setting to add a rel="nofollow" attribute to the links in comments, or do so by default. This may discourage some types of spam, but it's definitely not the only measure you should take.

Consider requiring users to create accounts before they can post a comment. This adds steps to the user experience and may discourage some casual visitors from posting comments, but may keep the signal-to-noise ratio higher as well.

Change your settings so that comments need to be approved before they show up on your site. This is a great tactic if you want to hold comments to a high standard, don't expect a lot of comments, or have a small, personal site. You may be able to allow employees or trusted users to approve posts themselves, spreading the workload.

Think about disabling some types of comments. For example, you may want to disable comments on very old posts that are unlikely to get legitimate comments. On blogs you can often disable trackbacks and pingbacks, which are very cool features but can be major avenues for automated spam.

Keep your site up-to-date

Take the time to keep your software up-to-date and pay special attention to important security updates. Some spammers take advantage of security holes in older versions of blogs, bulletin boards, and other content management systems. Check the Quick Security Checklist for additional measures.

You may need to strike a balance on which tactics you choose to implement depending on your blog or bulletin board software, your user base, and your level of experience. Opening up a site for comments without any protection is a big risk, whether you have a small personal blog or a huge site with thousands of users. Also, if your forum has been completely filled with thousands of spam posts and doesn't even show up in Google searches, you may want to submit a reconsideration request after you clear out the bad content and take measures to prevent further spam.

As a long-time blogger and web developer myself, I can tell you that a little time spent setting up measures like these up front can save you a ton of time and effort later. I'm new to the Webmaster Central team, originally from Cleveland. I'm very excited to help fellow webmasters, and have a passion for usability and search quality (I've even done a bit of academic research on the topic). Please share your tips on preventing comment and forum spam in the comments below, and as always you're welcome to ask questions in our discussion group.